I need to see more than one 1000 yard season to see how good David Montgomery is. There have been literally hundreds of NFL RBs that have had 1 or 2 decent seasons and then fade away. Raymont Harris, Anthony Thomas, Jordan Howard, Rashaan Salaam, and Jonathan Allen are examples for just the Bears. Others are Charles White, Steve Slaton, Olandis Gary, Mike Anderson, Ickey Woods, Peyton Hillis, Bobby Humphrey, Lamar Smith, Barry Foster, Natrone Means, and the list goes on and on. This is not uncommon for the RB position.
David Montgomery has great contact balance. He's shifty and can gain yards after contact. He has functional speed, but he does not have the speed to be an elite back. I have seen many of his big plays that would have been taken to the house if it were Matt Forte or Neal Anderson. Instead, David Montgomery is caught from behind. He hits the hole hard, but there's plenty of other backs that hit it faster. He has good hands as a receiver, but that lack of speed will prevent him from running the full WR route tree like Faulk or Anderson if he motions out wide.
It has been shown that he needs the line to play at a high level to succeed, which means he lacks the ability to create on his own like the greats such as Walter Payton or Barry Sanders. If Sweetness had the 85 offensive line for the entirety of his career instead of the crap he had in the 70s, he'd have over 20,000 career yards. He would have had an unbeatable record. If Barry Sanders played behind the Dallas O-Line of that time, he'd have 2500 yards in a season easy, and maybe even 3000.
David Montgomery is a good back, and I hope he develops into a great back. But, neither is going to happen unless the line plays solid.